The present invention relates to a novel rubber composition or, more particularly, to a rubber composition based on a blend of a rubbery copolymer of ethylene and propylene and an organopolysiloxane which is outstandingly free from the problem of stain on the surface of a metal mold in which the rubber composition is compression-molded and vulcanized.
It is a conventional process that a rubber composition is molded and shaped into a vulcanized rubber article by compression molding and simultaneous vulcanization in a metal mold at an elevated temperature under a high compressive force. One of the serious problems which badly influences the productivity of such a rubber vulcanizing process is that the surface of the metal mold is susceptible to stain originating in the rubber composition molded therein so that the rubber shaping process must be frequently interrupted for the cleaning work of the metal mold because, when a metal mold having surface stain is used for rubber molding, the vulcanized rubber article has poor surface gloss unacceptable as a commercial product. Accordingly, it is very important that rubber compositions are formulated so as to decrease the deposition of stain on the metal mold as far as possible from the standpoint of productivity in view of the large consumption of labor and time for the metal mold cleaning.
Although the above mentioned problem of stain on the metal mold is old, it is still unclear in what mechanism such a stain deposition takes place on the surface of the metal mold. Despite the difficult nature of the problem, the progress of the analytical technology in recent years is expected to provide an analytical means for the direct inspection of the entity of stain on the metal mold and some successful results have already been obtained for the constituents of the stain material indicating that the constituents in a rubber composition having the largest responsibility for the surface stain of the metal mold are mainly the degradation products of the rubber composition and the aging retarder always contained in the composition.